Plasmonic Elastic Capsules as Colorimetric Reversible pH-Microsensors.

There is a crucial need for effective and easily dispersible colloidal microsensors able to detect local pH changes before irreversible damages caused by demineralization, corrosion, or biofilms occur. One class of such microsensors is based on molecular dyes encapsulated or dispersed either in polymer matrices or in liquid systems exhibiting different colors upon pH variations. They are efficient but often rely on sophisticated and costly syntheses, and present significant risks of leakage and photobleaching damages, which is detrimental for mainstream applications. Another approach consists of exploiting the distance-dependent plasmonic properties of metallic nanoparticles. Still, assembling nanoparticles into dispersible colloidal pH-sensitive sensors remains a challenge. Here, it is shown how to combine optically active plasmonic gold nanoparticles and pH-responsive thin shells into "plasmocapsules." Upon pH change, plasmocapsules swell or shrink. Concomitantly, the distance between the gold nanoparticles embedded in the polymeric matrix varies, resulting in an unambiguous color change. Billions of micron-size sensors can thus be easily fabricated. They are nonintrusive, reusable, and sense local pH changes. Each plasmocapsule is an independent reversible microsensor over a large pH range. Finally, their potential use for the detection of bacterial growth is demonstrated, thus proving that plasmocapsules are a new class of sensing materials.

Plasmonic-Based Mechanochromic Microcapsules as Strain Sensors.

Efficiently detecting mechanical deformations within materials is critical in a wide range of devices, from micro-electromechanical systems to larger structures in the aerospace industry. This communication reports the fabrication of new mechanochromic micrometer-size capsules enabling the detection of strains. These microcapsules are synthesized using an emulsification approach. They are made of densely packed gold nanoparticles embedded in a spherical silica crust. Billions of these composite spherical microcapsules are fabricated in a single batch. Each microcapsule is an opto-mechanosensor by itself, and can easily be recovered and incorporated into polymer films. When the films are stretched, the microcapsules are deformed into elongated ellipsoidal shapes and the distance between the Au NPs embedded in their shells concomitantly increases. As the extinction of Au NPs depends on the separation between the Au NPs, microcapsules exhibit different colors when they are elongated. These novel sensitive microcapsules can be used to detect and measure strain in polymer films by outputting color information.

Physical mechanisms driving the reversible aggregation of Staphylococcus aureus and response to antimicrobials.

Formation of non-sessile, auto-aggregated cells of Staphylococcus aureus contributes to surface colonization and biofilm formation, hence play a major role in the early establishment of infection and in tolerance to antimicrobials. Understanding the mechanism of aggregation and the impact of aggregation on the activity of antimicrobials is crucial in achieving a better control of this important pathogen. Previously linked to biological phenomena, physical interactions leading to S. aureus cellular aggregation and its protective features against antimicrobials remain unraveled. Herein, in-vitro experiments coupled with XDLVO simulations reveal that suspensions of S. aureus cells exhibit rapid, reversible aggregation (> 70%) in part controlled by the interplay between cellular hydrophobicity, surface potential and extracellular proteins. Changing pH and salt concentration in the extracellular media modulated the cellular surface potential but not the hydrophobicity which remained consistent despite these variations. A decrease in net cellular negative surface potential achieved by decreasing pH or increasing salt concentrations, caused attractive forces such as the hydrophobic and cell-protein interactions to prevail, favoring immediate aggregation. The aggregation significantly increased the tolerance of S. aureus cells to quaternary ammonium compounds (QAC). The well-dispersed cell population was completely inactivated within 30 s whereas its aggregated counterpart required more than 10 min.